Kerberos provider

**Describe the idea**

Implement a Kerberos provider. authentik would act as a KDC, allowing users to get keytabs for services.

I've already laid out some ground work at https://github.com/rissson/authentik/tree/kerberos-as-proxy, but that's very much a WIP.

The idea behind that implementation is that authentik would receive KDCProxy requests at `/application/k/my_app/proxy/`, decode them and process the request from there. Technically, it could be implemented as an outpost, but I don't want to do that as we need to make some database access (as the user's keytab is retrieved multiple times during authentication), and because I don't want to write Go for fun.

If you want to help working on this idea or want to contribute in any other way, react to this issue with a :rocket: